Musannaf Ibn Abi Shayba 36:335

Waki’ narrated to us, he said: Sa’id ibn ‘Abd al-‘Aziz al-Tanukhi narrated to us, from Makhul, from Zayd ibn Jariya, from Habib ibn Maslama, he said: I witnessed the Prophet, may Allah bless him and grant him peace, granting a third as a voluntary gift (nafl).