Verse 77: فَأَعْقَبَهُمْ نِفَاقًا فِي قُلُوبِهِمْ (So Allah, in turn, put hypocrisy in their hearts) points out to its cause which lies in their lying and pledge breaking. As a consequence, Allah made the hypocrisy of their hearts become deep and firm. Now, they would just not have the ability to make Taubah itself.
A serious note of warning
From here we learn that there are occasions when the curse of evil deeds assumes alarming proportions, so much so that one is deprived of the very ability (taufiq) of making Taubah. May Allah protect us from this misfortune!
Let us now go back to the detailed narrative of Sayyidna Abu Umamah ؓ mentioned a little earlier. Ibn Jarir writes at the end of it: When the Holy Prophet ﷺ said Was for Tha'labah' three times, some of his relatives were present in the gathering. When they heard it, one of them immediately traveled to see Tha’ labah. When he reached there, he reproached him for his behavior while informing him that it has caused the revelation of a particular verse of the Qur'an. This made Tha'labah nervous. He reached Madinah and requested the Holy Prophet ﷺ that his Sadaqah may be accepted. He said that Allah Ta` ala had told him not to accept his Sadaqah. Hearing this, Tha'labah went crazy with disappointment and literally started throwing dust on his head.
The Holy Prophet ﷺ said, ` this is something you have chosen to do on your own. I ordered you and you did not obey. Now, your Sadaqah cannot be accepted.' Tha'labah returned disappointed. Then, some days later, the Holy Prophet ﷺ departed from this mortal world and Sayyidna Abu Bakr ؓ became the Khalifah. Tha'labah came to Sayyidna Abu Bakr ؓ and requested that he should accept his sadaqah. Sayyidna Abu Bakr ؓ said, ` when the Holy Prophet ﷺ did not accept it, how can I do it?'
Then, after the demise of Sayyidna Abu Bakr ؓ Tha'labah came to Sayyidna ` Umar ؓ عن . He made the same request and got the same answer from him, as was given by Sayyidna Abu Bakr ؓ عن . Again, he submitted this request to Sayyidna Uthman ؓ during his period of Khilafah. He too refused it. It was during the tenure of the Khilafah of Sayyidna Uthman ؓ ، that Tha'labah died. We seek the protection of Allah from all evil deeds. (Mazhari)
A question and its answer
When Tha'labah had submitted in repentance, the question is why was his Taubah not accepted? The reason is evident. The Holy Prophet ﷺ was informed through revelation that he was not making his Taubah with absolute sincerity. He had hypocrisy concealed in his heart. He was simply trying to deceive Muslims for the time being only to put matters right between them. Therefore, it was not acceptable. And when the Holy Prophet I himself declared him to be a hypocrite, the Khulafa' who succeeded him were left with no right to accept his Sadaqah - because, being a Muslim is a condition for Zakah. Now, after the Holy Prophet JI no one knows the hypocrisy hidden in the heart of a person, therefore, the rule to follow in future is: Anyone who makes Taubah and confesses to his Islam and 'Iman should be treated as Muslims are treated - no matter what lies in his heart. (Bayn a1-Qur'n)