It was narrated from 'Ubaidullah bin 'Abdullah bin 'Umar that his father said:
"The Messenger of Allah said: 'If the water is the amount of two or three Qullah, nothing can make it impure (Najis).'" (Sahih)Another chain with similar wording. (DE)
English reference (DE): Vol. 1, Book 1, Hadith 518 (DE)
Arabic reference (DE): Book 1, Hadith 559 (DE)