ENOfficial
Nafi' said, "Ibn 'Umar needed something from Mu'awiya and he wanted to write to him. People said, 'Begin with his name.' They kept on at him until he wrote, 'In the Name of Allah, the All-Merciful, Most Merciful, to Mu'awiya.'"
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا قُتَيْبَةُ، قَالَ: حَدَّثَنَا يَحْيَى بْنُ زَكَرِيَّا، عَنِ ابْنِ عَوْنٍ، عَنْ نَافِعٍ قَالَ: كَانَتْ لِابْنِ عُمَرَ حَاجَةٌ إِلَى مُعَاوِيَةَ، فَأَرَادَ أَنْ يَكْتُبَ إِلَيْهِ، فَقَالُوا: ابْدَأْ بِهِ، فَلَمْ يَزَالُوا بِهِ حَتَّى كَتَبَ: بِسْمِ اللهِ الرَّحْمَنِ الرَّحِيمِ، إِلَى مُعَاوِيَةَ.
sunnah.com