This one was tough to find a reference to, but it looks like this is the hadith being referred to:
عَنْ أَبِي الدَّرْدَاءِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " أَلاَ أُخْبِرُكُمْ بِأَفْضَلَ مِنْ دَرَجَةِ الصِّيَامِ وَالصَّلاَةِ وَالصَّدَقَةِ " . قَالُوا بَلَى . قَالَ " إِصْلاَحُ ذَاتِ الْبَيْنِ وَفَسَادُ ذَاتِ الْبَيْنِ الْحَالِقَةُ "
The Prophet (saws) said: “Do you want me to tell you of what is better than the rank of fasting, prayer and charity?” The Companions said, “Yes, O Messenger of Allaah. He said, “Making peace between people among whom there is discord, for the evil of discord is the shaver.”
This is in Sunan Abi Dawud 4919. There is another version in Jami` at-Tirmidhi 2509 which adds that the Prophet (saws) said
“It is the shaver, and I do not say that it shaves hair, but that it shaves (i.e., destroys) the deen (i.e. religious commitment).”
The version in Jami` Tirmidhi was authenticated as sahih by Shaykh al-Albani.
According to the translation on islamqa this refers to voluntary acts of fasting, prayer, and charity. Which makes sense because the obligatory versions are nothing less than the pillars of Islam.