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1) Milinda – Panha : 426-432; Middle length Sayings 2 : 97-101
2) Samyutta Nikaaya-3 : 115-117; Kindred Sayings : 99-101

The questions were asked by Malunkyaputta.

When Malunkyaputta asked these questions, Lord Buddha said “these questions are meaningless and not relevant to the amelioration (solution to) of suffering.”
Lord Buddha gave the example that when an arrow strikes a person, there is no use of investigating as to who shot the arrow? What is the substance with which the arrow is made? Etc.

Why Lord Buddha did not answer?
Lord Buddha did not answer the questions does not mean that He did not know the answer. He is the incarnation of the Supreme Lord Sri Krishna who is the source of all vedas (vedais ca sarvair aham eva vedya…BG 15.15)

In the third chapter of the first canto of Srimad Bhagavatam, while describing the various incarnations of the Lord, Srila Suta gosvami mentions about Lord Buddha in SB 1.3.24.
The verse is SB 1.3.24.
tataḥ kalau sampravṛtte / sammohāya sura-dviṣām
buddho nāmnāñjana-sutaḥ / kīkaṭeṣu bhaviṣyati
in this verse, the words “sammohaaya sura dvisaam” indicate that Lord Buddha appeared to delude the atheists.

Below is the excerpt from the purport of Srila Prabhupada to Srimad Bhagavatam 1.3.24. In this Purport Srila Prabhupada answers all your questions.

“Technically Lord Buddha’s philosophy is called atheistic because there is no acceptance of the Supreme Lord and because that system of philosophy denied the authority of the Vedas.
But that is an act of camouflage by the Lord. Lord Buddha is the incarnation of Godhead. As such, he is the original propounder of Vedic knowledge. He therefore cannot reject Vedic philosophy. But he rejected it outwardly because the sura-dviṣa, or the demons who are always envious of the devotees of Godhead, try to support cow-killing or animal-killing from the pages of the Vedas, and this is now being done by the modernized sannyāsīs. Lord Buddha had to reject the authority of the Vedas altogether. This is simply technical, and had it not been so he would not have been so accepted as the incarnation of Godhead. Nor would he have been worshiped in the transcendental songs of the poet Jayadeva, who is a Vaiṣṇava ācārya. Lord Buddha preached the preliminary principles of the Vedas in a manner suitable for the time being (and so also did Śańkarācārya) to establish the authority of the Vedas. Therefore both Lord Buddha and Ācārya Śańkara paved the path of theism, and Vaiṣṇava ācāryas, specifically Lord Śrī Caitanya Mahāprabhu, led the people on the path towards a realization of going back to Godhead.”