‘Abdallah b. Mas'ud said: I was praying, and so was the Prophet (ﷺ) accompanied by Abu Bakr and ‘Umar. When I sat I first extolled God’s praises then invoked blessings on the Prophet, then made supplication for myself, and the Prophet (ﷺ) said, “If you ask you will be given it, if you ask you will be given it.” Tirmidhi transmitted it.
Source: sunnah.com
Other translations
Arabic
وَعَنْ عَبْدِ اللَّهِ بْنِ مَسْعُودٍ قَالَ: كُنْتُ أُصَلِّي وَالنَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ وَأَبُو بَكْرٍ وَعُمَرُ مَعَهُ فَلَمَّا جَلَسْتُ بَدَأْتُ بِالثَّنَاءِ عَلَى اللَّهِ تَعَالَى ثُمَّ الصَّلَاةُ عَلَى النَّبِيِّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ ثُمَّ دَعَوْتُ لِنَفْسِي فَقَالَ النَّبِيُّ صَلَّى اللَّهُ عَلَيْهِ وَسَلَّمَ: «سَلْ تعطه سل تعطه» . رَوَاهُ التِّرْمِذِيّ
sunnah.com