It was narrated that Ibrahim said:"They used to say: 'If one is not sure of what he estimates is correct, then prostrate twice
Hadith No. 1247
Sunan an Nasai
Gradings:
Abu Ghuddah:
Sahih Isnaad Mauquf
Al-Albani:
Sahih Isnaad Mauquf
Zubair Ali Zai:
Isnaad Sahih