Belief and Action
The Prophet(s) instructs the man “Say: I believe in Allah, then be steadfast” as there is some value in him outwardly stating and restating his belief in Allah. It’s not enough to just say, but to actually follow through with action. Many times, we are very good at talking, but very weak at doing, practicing, and living by our words. Simply saying by the tongue does not absolve us of our responsibility of living by the words we say/proclaim (say or proclaim). The following verse in the Qur’an emphasizes this point:

إِنَّ ٱلَّذِينَ قَالُوا۟ رَبُّنَا ٱللَّهُ ثُمَّ ٱسْتَقَٰمُوا۟ تَتَنَزَّلُ عَلَيْهِمُ ٱلْمَلَٰٓئِكَةُ أَلَّا تَخَافُوا۟ وَلَا تَحْزَنُوا۟ وَأَبْشِرُوا۟ بِٱلْجَنَّةِ ٱلَّتِى كُنتُمْ تُوعَدُونَ

“Indeed, those who have said, ‘Our Lord is Allah’ and then remained on a right course - the angels will descend upon them, [saying], ‘Do not fear and do not grieve but receive good tidings of Paradise, which you were promised.’” (Qur’an, Surah Fussilat, 41:30)

Umar bin al-Khattab (may Allah be pleased with him) said, “They are steadfast upon the obedience of Allah, and they do not swerve away from that with the evasions of the fox.” This means they are firm upon what Allah has ordered from matters of belief, speech, and action. In addition, they consistently persist upon these orders of Allah.

إِنَّ ٱلَّذِينَ قَالُوا۟ رَبُّنَا ٱللَّهُ ثُمَّ ٱسْتَقَٰمُوا۟ فَلَا خَوْفٌ عَلَيْهِمْ وَلَا هُمْ يَحْزَنُونَ

“Indeed, those who have said, ‘Our Lord is Allah’, and then remained on a right course - there will be no fear concerning them, nor will they grieve.” (Qur’an, Surah al-Ahqaf, 46:13)