Musannaf Ibn Abi Shayba 3:461

Abu Bakr narrated to us, he said: Ibn Fudayl narrated to us, from 'Umarah ibn al-Qa'qa', from Abu Zur'ah, from Abu Hurayrah, who said: When the Prophet (peace and blessings of Allah be upon him) would say the Takbir, he would remain silent between the Takbir and the recitation.