When did Imam Abu Hanifa begin teaching and issuing fatwas?

Imam Abu Hanifa al-Nu'man ibn Thabit, a great jurist and imam, some narrations indicate that he undertook giving fatwas and teaching after the death of his shaykh Hammad ibn Abi Sulayman. However, these narrations have weak chains of transmission due to the presence of liars among their narrators.

In contrast, it has been authentically narrated that Abu Hanifa remained with Hammad ibn Abi Sulayman for eighteen years until his death, and that he began giving fatwas and teaching after the death of his shaykh Hammad ibn Abi Sulayman, at which time he was forty years old.