ENOfficial
Amir b. Sa'd reported on the authority of his father that Allah's Messenger (ﷺ) said: The greatest sinner amongst the Muslims is one who asked about a thing (from Allah's Messenger) which had not been forbidden for the Muslims and it was forbidden for them because of his persistently asking about it.
Source: sunnah.com
Other translations
Arabic
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، أَخْبَرَنَا إِبْرَاهِيمُ بْنُ سَعْدٍ، عَنِ ابْنِ شِهَابٍ، عَنْ عَامِرِ بْنِ، سَعْدٍ عَنْ أَبِيهِ، قَالَ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم " إِنَّ أَعْظَمَ الْمُسْلِمِينَ فِي الْمُسْلِمِينَ جُرْمًا مَنْ سَأَلَ عَنْ شَىْءٍ لَمْ يُحَرَّمْ عَلَى الْمُسْلِمِينَ فَحُرِّمَ عَلَيْهِمْ مِنْ أَجْلِ مَسْأَلَتِهِ " .
sunnah.com