The sutta is DN 16, where the Buddha is giving his last instructions. He gives four injunctions in total:
Now the Blessed One spoke to the Venerable Ananda, saying: "It may be, Ananda, that to some among you the thought will come: 'Ended is the word of the Master; we have a Master no longer.' But it should not, Ananda, be so considered. For that which I have proclaimed and made known as the Dhamma and the Discipline, that shall be your Master when I am gone.
"And, Ananda, whereas now the bhikkhus address one another as 'friend,' let it not be so when I am gone. The senior bhikkhus, Ananda, may address the junior ones by their name, their family name, or as 'friend'; but the junior bhikkhus should address the senior ones as 'venerable sir' or 'your reverence.'
"If it is desired, Ananda, the Sangha may, when I am gone, abolish the lesser and minor rules.
"Ananda, when I am gone, let the higher penalty be imposed upon the bhikkhu Channa."
"But what, Lord, is the higher penalty?"
"The bhikkhu Channa, Ananda, may say what he will, but the bhikkhus should neither converse with him, nor exhort him, nor admonish him."
It's an interesting allowance, given what he says earlier in the same sutta:
"The growth of the bhikkhus is to be expected, not their decline, bhikkhus, so long as they ... appoint no new rules, and do not abolish the existing ones, but proceed in accordance with the code of training (Vinaya) laid down;
The vinaya (Culavagga) gives an account of the sangha's decision in regards to this allowance:
Then the venerable Ânanda spake thus to the Thera Bhikkhus: 'The Blessed One, Sirs, at the time of his passing away, spake thus to me "When I am gone, Ânanda, let the Samgha, if it should so wish, revoke all the lesser and minor precepts."'
'Did you then, venerable Ânanda, ask the Blessed One which were the lesser and minor precepts?'
Some Theras then said that all the rules save the four Pârâgikas; others that all save those and the thirteen Samghâdisesas; others that all save those and the two Aniyatas; others that all save those and the thirty Nissaggiyas; others that all save those and the ninety-two Pâkittiyas; others that all save those and the four Pâtidesaniyas were lesser and minor precepts.
Then the venerable Mahâ Kassapa laid a resolution before the Samgha: 'Let the venerable Samgha hear me. There are certain of our precepts which relate to matters in which the laity are concerned. Now the laity know of us that "such and such things are proper for you Samanas who are Sakyaputtiyas, and such and such things are not." If we were to revoke the lesser and minor precepts, it will be said to us: "A set of precepts was laid down for his disciples by the Samana Gotama to endure until the smoke should rise from his funeral pyre. So long as their teacher remained with these men, so long did they train themselves in the precepts. Since their teacher has passed away from them, no longer do they now train themselves in the precepts.'
'If the time seems meet to the Samgha, not ordaining what has not been ordained, and not revoking what has been ordained, let it take upon itself and ever direct itself in the precepts according as they have been laid down. This is the resolution.
'Let the venerable Samgha hear me. [These things being so] the Samgha takes upon itself the precepts according as they have been laid down. Whosoever of the venerable ones approves thereof, let him keep silence. Whosoever approves not thereof, let him speak. The Samgha has taken upon itself the precepts according as they were laid down. Therefore does it keep silence. Thus do I understand.'
It is considered meaningful what language the Buddha used to give his allowance ("If it is desired"). The commentary to DN 16, picking up on this, explains the whole issue as a means of showing Maha-Kassapa's strength in leading the sangha to the right decision. The sub-commentary elaborates, explaining it as a means of showing that the sangha followed the Buddha's rules because they wanted to, not because they were forced to.
The commentary also comments on the issue of Nagasena's decisive answer (in the Milindapanha) in regards to what really constitutes "lesser and minor rules" (according to him it was the dukkata and dubhasita offences). It explains that some say Maha-Kassapa didn't know, and that was the reason why he decided they should keep them all. The commentary says this view should not be entertained; that Nagasena gave a decisive answer because he didn't want do give opportunity for debate, whereas Maha-Kassapa simply didn't want to abolish the lesser and minor rules (khuddānukhuddakāpattiṃ na samūhanissāmi).
Either way, the decision reached by the first council with Maha-Kassapa as its head is to be considered authoritative, until and unless someone decides to revoke that sangha-kamma.