It is narrated from Sayyiduna Bilal (may Allah be pleased with him) that on Friday he used to give the adhan for the Messenger of Allah (peace and blessings be upon him) when the shadow had become the length of a sandal strap, when the Prophet (peace and blessings be upon him) sat on the pulpit.
Hadith Referenceالمعجم الكبير للطبراني / باب الباء / 1071