Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) said that Zayd bin Harithah was the freed slave of the Prophet (peace and blessings be upon him); we used to call him "Zayd bin Muhammad" until the Qur'an was revealed: "Call them by (the names of) their fathers; that is more just in the sight of Allah." [Surah al-Ahzab: 5]
Hadith Referenceالسنن الكبرى للبيهقي / كتاب النكاح / 13914