It is narrated from Sayyiduna Ubadah bin al-Samit (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "If it pleases you to wear a collar of fire, then take a wage for teaching writing."
Ali bin al-Madini (may Allah have mercy on him) said: Its chain is entirely known except for al-Aswad bin Tha'labah, for we know nothing from him except this hadith.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الإجارة / 11682