Musnad Ahmad 27:164

Yahya ibn Sa'id narrated to us, from 'Ubayd Allah, Nafi' narrated to me, from Ibn 'Umar, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "The month is twenty-nine [days] like this, and like this, and like this. If it is obscured from you, then estimate it." He said: And Ibn 'Umar used to, if it was the night of the twenty-ninth and there were clouds or haze in the sky, wake up fasting.