It is narrated from Sayyiduna Jabir bin Abdullah (may Allah be pleased with them both) with the same chain that the flag of the Noble Prophet (peace and blessings be upon him) was black in color.
Hadith Referenceمعجم صغير للطبراني / كتاب الجهاد / 599
Hadith Gradingمحدثین:إسناده صحيح
Hadith Takhrij«إسناده صحيح ، وأخرجه الطبراني فى«الكبير» برقم: 1758، والطبراني فى «الأوسط» برقم: 7969، والطبراني فى «الصغير» برقم: 1077 قال الهيثمي: في إسناده شريك النخعي وثقه النسائي وغيره وفيه ضعف وبقية رجاله ثقات ، مجمع الزوائد ومنبع الفوائد: (5 / 321)»
´It was narrated from Jabir that:` the Prophet enter Makkah and his standard was white.
Hafiz Muhammad Ameen
This pertains to the conquest of Makkah; therefore, a flag was necessary. Otherwise, on the occasion of the Farewell Pilgrimage (Hajjat al-Wada‘), there was no flag or the like. In some narrations, it is mentioned that the flag of the Prophet (sallallahu alayhi wa sallam) was black. There is no contradiction in this. The main flag of the army was black, and the personal flag of the Prophet (sallallahu alayhi wa sallam) was white. In any case, there are several flags in war. Even during the conquest of Makkah, the Muhajirun had a separate flag, the Ansar had a separate one, and likewise for other groups.
Source: Sunan Nasa'i: Translation and Benefits by Shaykh Hafiz Muhammad Amin Hafizullah, Page: 2869