It has been narrated on the authority of A'isha, wife of the Prophet (ﷺ), who said:

The Messenger of Allah (ﷺ) set out for Badr. When he reached Harrat-ul-Wabara (a place four miles from Medina) a man met him who was known for his valour and courage. The Companions of the Messenger of Allah (ﷺ) were pleased to see him. He said: I have come so that I may follow you and get a share from the booty. The Messenger of Allah (ﷺ) said to him: Do you believe in Allah and His Messenger? He said: No. The Messenger of Allah (ﷺ) said: Go back, I will not seek help from a Mushrik (polytheist). He went on until we reached Shajara, where the man met him again. He asked him the same question again and the man gave him the same answer. He said: Go back. Im will not seek help from a Mushrik. The man returned and overtook him at Baida'? He asked him as he had asked previously: Do you believe in Allah and His Messenger? The man said: Yes. The Messenger of Allah (ﷺ) said to him: Then come along with us.

حَدَّثَنِي زُهَيْرُ بْنُ حَرْبٍ، حَدَّثَنَا عَبْدُ الرَّحْمَنِ بْنُ مَهْدِيٍّ، عَنْ مَالِكٍ، ح وَحَدَّثَنِيهِ أَبُو الطَّاهِرِ - وَاللَّفْظُ لَهُ - حَدَّثَنِي عَبْدُ اللَّهِ بْنُ وَهْبٍ، عَنْ مَالِكِ بْنِ أَنَسٍ، عَنِ الْفُضَيْلِ بْنِ أَبِي، عَبْدِ اللَّهِ عَنْ عَبْدِ اللَّهِ بْنِ نِيَارٍ الأَسْلَمِيِّ، عَنْ عُرْوَةَ بْنِ الزُّبَيْرِ، عَنْ عَائِشَةَ، زَوْجِ النَّبِيِّ صلى الله عليه وسلم أَنَّهَا قَالَتْ خَرَجَ رَسُولُ اللَّهِ صلى الله عليه وسلم قِبَلَ بَدْرٍ فَلَمَّا كَانَ بِحَرَّةِ الْوَبَرَةِ أَدْرَكَهُ رَجُلٌ قَدْ كَانَ يُذْكَرُ مِنْهُ جُرْأَةٌ وَنَجْدَةٌ فَفَرِحَ أَصْحَابُ رَسُولِ اللَّهِ صلى الله عليه وسلم حِينَ رَأَوْهُ فَلَمَّا أَدْرَكَهُ قَالَ لِرَسُولِ اللَّهِ صلى الله عليه وسلم جِئْتُ لأَتَّبِعَكَ وَأُصِيبَ مَعَكَ قَالَ لَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ تُؤْمِنُ بِاللَّهِ وَرَسُولِهِ ‏"‏ ‏.‏ قَالَ لاَ قَالَ ‏"‏ فَارْجِعْ فَلَنْ أَسْتَعِينَ بِمُشْرِكٍ ‏"‏ ‏.‏ قَالَتْ ثُمَّ مَضَى حَتَّى إِذَا كُنَّا بِالشَّجَرَةِ أَدْرَكَهُ الرَّجُلُ فَقَالَ لَهُ كَمَا قَالَ أَوَّلَ مَرَّةٍ فَقَالَ لَهُ النَّبِيُّ صلى الله عليه وسلم كَمَا قَالَ أَوَّلَ مَرَّةٍ قَالَ ‏"‏ فَارْجِعْ فَلَنْ أَسْتَعِينَ بِمُشْرِكٍ ‏"‏ ‏.‏ قَالَ ثُمَّ رَجَعَ فَأَدْرَكَهُ بِالْبَيْدَاءِ فَقَالَ لَهُ كَمَا قَالَ أَوَّلَ مَرَّةٍ ‏"‏ تُؤْمِنُ بِاللَّهِ وَرَسُولِهِ ‏"‏ ‏.‏ قَالَ نَعَمْ ‏.‏ فَقَالَ لَهُ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ فَانْطَلِقْ ‏"‏ ‏.‏