It is reported from Sayyiduna Abdullah (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) said: "Seeking lawful earnings is an obligation after the obligation."
Only Abbad bin Kathir al-Ramli narrated it, and he is da'if (weak). When Yahya bin Yahya (may Allah have mercy on him) was asked about the hadith of Abbad bin Kathir on lawful earnings, "The Messenger of Allah (peace and blessings be upon him) said", he said: If he (really) said it.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11695