Ven. Sir. most likely Apacāyana is a word used in the commentaries more particularly appearing Abhidhamma commentaries like Attha,sālinī, Abhidhamm’attha,saṅgaha, Abhidhamm’attha,vibhāvinī, Moha,vicchedanī. (Search for Apacāyana within the Pali Canon returns nothing while other sources are mentioned in the background and introduction in Puñña,kiriya,vatthu Sutta compiled by Piya Tan)
The salient point with regard to practicing Apacāyana is that you dissolve the ego, pride and self-righteousness, within there is the perception of "I". In addition there is sense of gratitude1 or respect for higher virtue or attainment in the person you are venerating2.
As for source I would like to point out the following:
Apacayana means paying respects to those who excel you in age, morality, integrity, wisdom, virtue, etc. Paying respects to elderly persons such as your father, mother, uncle, aunt; offering your seat and making way for those worthy of respect; bowing your head and showing humility, clasping your palms in homage to Bhikkhus, doffing your hat, saluting according to custom, etc. are all signs of respect. However, if you show respect unwillingly to a powerful person out of fear or with some selfish aim, this cannot be called apacayana, because it is pretentious in nature. It only amounts to maya (trickery).
Note: Food for thought - bowing or curtseying is generally accepted as signs of reverence. In Myanmar some people put down whatever load they are carrying and prostate on the roads when they meet Bhikkhus. Some kneel down in the sidewalk or on the platform of a railway station to pay respects to monks and elderly persons. These actions if done with true sincerity, are not to be blamed. But in these days when people have to rush about in busy places, just a bow or a few humble words will suffice the need of apayacana. Kneeling down and prostrating in worshiping on meeting a Bhikkhu on the roads in a Bhikkhu on the roads or in busy crowded places in the presence of alien people are not really necessary.
Source: Abhidhamma In Daily Life / Chapter 6 - Ten Domains Of Meritorious Actions / Domain 4 - Apacayana
Respect as a way of making merit should be known in such acts as getting up from
one’s seat, welcoming one’s mother, father, elder brother, elder sister or an elderly person,
taking his/her luggage, saluting him, showing him the way, and so on. Or generally,
respecting others’ feelings, privileges, property, and life; regarding them with deference,
esteem and honour; avoiding degrading, insulting or interrupting them; refraining
from offending, corrupting or tempting them. Sadly, today the younger generation
lacks respect or reverence.
According to Venerable Nāgasena, in Milindapañha, there are twelve persons who do
not pay respect or show reverence to others: a lustful person because of his lust; an angry person because of his anger; a confused person because of his confusion; an arrogant
person because of his pride; one devoid of special qualities owing to his lack of
distinction; an obstinate owing to his lack of docility; a low minded owing to his low
mindedness; an evil man owing to his selfishness; an afflicted owing to his affliction; a
greedy owing to his being overcome by greed; and a businessman owing to his working
It is clear that, in contrast to the above persons, the reverent and respectful man develops
his mind (and thereby accumulates merit), for by his attitude he cuts down the defilement
of pride and replaces it by wise conduct of humility. Respecting elders and the
Saïgha are clear examples of this aspect. Even the respect shown by a novice monk to a
bhikkhu falls under this category.
Here again, the good intention that arises in one who shows respect or reverence is the
way of making merit in respecting others.
Source: TEN WAYS OF MAKING MERIT by MAHINDA WIJESINGHE
Respectfulness (apacāyana) is the means by which one shows respect (apacayati), acts properly
by way of honouring. Traditionally, respect is shown in such ways as when we meet a practising monk,
salute him with joined palms, we take his bowl and robe, and offer him a seat and some water as appropriate.
A common mark of respect would be to give way to elders when meeting them. “Respect” here
should not be taken in the ritual way, which has very little moral value, as the intention behind such an
action is not always pure. “Respect” properly means “accepting others as they are,” that is, using our present-moment
awareness not to measure or judge others, but to relate to them (especially those near and
dear) as if meeting them for the first time. In other words, when we show a sincere interest in people, we
are more likely to encourage them to show their better sides. There is also “self-respect,” that is, abstaining
from evil or unwholesome deeds through the feeling of moral shame (ottappa), that such negative
results would bring disgrace upon us and those close to us.
Source: Puñña,kiriya,vatthu Sutta commentary and analysis by Piya Tan
Since gāravā is a loose equivalent as per the comments the following are the Suttas which has this word: search for gāravā within the Pali Canon.
1 Especially to those who helped in learning and coming in contact with the Dhamma, especially you teacher, the lineage of Sangha who preserved the teaching, the Buddha as if it was not for the Buddha we will have not come across the Dhamma at all and our Parami is not strong enough to realise it, and the Dhamma itself as even the Buddha kept it as his teacher.
2 Garava Sutta