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. (IT)
English reference (IT): Vol. 1, Book 1, Hadith 518 (IT)
Arabic reference (IT): Book 1, Hadith 559 (IT)