Musnad Ahmad

It is narrated from Hazrat Adi (may Allah be pleased with him) that the Holy Prophet (peace and blessings be upon him) taught me about prayer and fasting and said, "Pray at such and such times, and fast. When the sun sets, eat and drink until the white thread becomes distinct and clear from the black thread before you." And keep thirty fasts unless the moon is sighted before that. So I took two threads, one black and one white, and placed them under a pillow. I kept looking at them, but the black thread did not become distinct from the white, nor the white from the black. When morning came, I presented myself before the Holy Prophet (peace and blessings be upon him) and related the whole incident. The Holy Prophet (peace and blessings be upon him) said, "Your pillow is indeed very wide; what is meant is the light of the day and the darkness of the night."