I could find the following references to the Sarasvati River in the Pali Canon:
Now at that time the brahmin Sundarika Bhāradvāja was sitting not far
from the Buddha. He said to the Buddha, “But does Master Gotama go to
the river Bāhuka to bathe?”
“Brahmin, why go to the river Bāhuka? What can the river Bāhuka do?”
“Many people agree that the river Bāhuka bestows cleanliness and
merit. And many people wash off their bad deeds in the river Bāhuka.”
Then the Buddha addressed Sundarika in verse:
The Bāhuka and the Adhikakka,
the Gaya and the Sundarika too,
Sarasvatī and Payāga,
and the river Bāhumati:
a fool can constantly plunge into them
but it won’t purify their dark deeds.
What can the Sundarika do?
What the Payāga or the Bāhuka?
They can’t cleanse a cruel and criminal person
from their bad deeds.
For the pure in heart it’s always
the spring festival or the sabbath.
For the pure in heart and clean of deed,
their vows will always be fulfilled.
It’s here alone that you should bathe, brahmin,
making yourself a sanctuary for all creatures.
And if you speak no lies,
nor harm any living creature,
nor steal anything not given,
and you’re faithful and not stingy:
what’s the point of going to Gaya?
For any well may be your Gaya!”
Majjhima Nikaya 7
Oh, when will I cross the Ganges, Yamunā,
and Sarasvatī rivers, the Pātāla country,
and the dangerous Baḷavāmukha sea,
by psychic power unimpeded? When will it be?
And again, O king, just as there are five hundred rivers which flow
down from the Himālayas, but of these ten only are reckoned in
enumerations of rivers—the Ganges, the Jumna, the Aciravatī, the
Sarabhū, the Mahī, the Indus, the Sarasvatī, the Vetravatī, the
Vītaṁsā, and the Candabhāgā—the others not being included in the
catalogue because of their intermittent flow of water.
The Indus, and the Sarasvatī
are rivers, like the Candabhāgā,
the Ganges and the Yamuna
the Sarabhu and the Mahī too.
Thera Apadana 3
There's another mention of this river in this Chinese Mahayana Sutra:
Nāgasena asked the king, “Would those who have never seen the great
ocean, know that the ocean is great? There are five great rivers, and
to each of them, there are also five hundred small tributaries. The
five great rivers are: (1) the Ganges, (2) the Sindhu, (3) the Sita,
(4) the Oxus and (5) the Sarasvati. The waters of these five great
rivers flow to the ocean day and night, but the water in the ocean
neither increases nor decreases.”
T 1670B Nāgasena Bhikṣu Sūtra 2.46