Hadith: Intercede and you will be rewarded, and Allah will decide what He loves on the tongue of His Prophet

عن أبي موسى الأشعري رضي الله عنه كان النبي صلى الله عليه وسلم إذا أتاه طالب حاجة أقبل على جُلسائِه، فقال: «اشْفَعُوا تُؤجَرُوا، ويَقْضِي الله على لسانِ نَبِيِّه ما أحب». وفي رواية: «ما شاء».
[صحيح] - [متفق عليه]

Abu Mūsa al-Ash‘ari (may Allah be pleased with him) reported: Whenever someone with a need came to the Prophet (may Allah's peace and blessings be upon him) he would turn to his companions and say: "Intercede (for him) and you will be rewarded, and Allah will decide what He loves on the tongue of His Prophet." In another narration: "What he wishes."
[Authentic hadith] - [Narrated by Bukhari & Muslim]

Explanation

This Hadīth sets a major principle and guides to a great benefit; it is that a believer should strive to help fulfill the need of others, whether his efforts bear fruit fully or partially or bear no fruit at all. One way to help people is to intercede on behalf of those who need things from the rulers or the dignitaries, or those who can fulfill their needs. Many people avoid to intercede for others if they are not sure whether their intercession would be accepted or not; hence, they miss much reward from Allah, and miss doing a favor to a fellow Muslim. Therefore, the Prophet (may Allah's peace and blessings be upon him) ordered his Companions to help the ones with need by interceding with him so that they receive their reward from Allah based on the Prophet's statement: "Intercede and you will receive reward." Good intercession is beloved to Allah; He says in the Quran: {Whoever intercedes for a good cause will have a reward therefrom} [Sūrat An-Nisā’: 85]. Along with hastening to receive the reward, the intercessor also hastens to do a favor to his Muslim brother, and that person will owe him. Striving to help people in good things that may or may not bear fruit is indeed an immediate goodness, and a way to train oneself to cooperate in righteousness and piety, and a start to intercede about things which are certain or likely to be fulfilled. The Prophet's statement: "And Allah will decide whatever He wishes on the tongue of His Prophet" means that Allah decrees what He willed in His eternal knowledge as to whether the interceded matter will take place or not. Hence, it is required to intercede and he will be rewarded for his intercession, whether the intercession is accepted or not.