Abu Dawud Book 2, Hadith Number 1522.
Chapter : On seeking pardon and repentance.
Narated By Abu Musa al-Asha’ri : They (the Companions) accompanied the Prophet (PBUH) while they were climbing the turning of a hill. A man uttered loudly: “There is no god but Allah, and Allah is most great,” when he ascended the hill. The Prophet of Allah (PBUH) said: You are not supplicating one who is deaf or absent. He then said: ‘Abd Allah b. Qais. The narrator then transmitted a tradition to the same effect.