Answer: This is Bud-Gumānī [unlawful suspicion] and unlawfully suspecting a pious Muslim is Ḥarām and leads to Hell. Allah عَزَّوَجَلَّ says in the 36th verse of Sūraĥ Banī Isrāīl (part 15):
وَ لَا تَقۡفُ مَا لَیۡسَ لَکَ بِہٖ عِلۡمٌ ؕ اِنَّ السَّمۡعَ وَ الۡبَصَرَ وَ الۡفُؤَادَ کُلُّ اُولٰٓئِکَ کَانَ عَنۡہُ مَسۡـُٔوۡ لًا ﴿۳۶﴾
And do not go after that thing which you do not know; indeed the ear, and the eye, and the heart – each of these will be questioned.
[Kanz-ul-Īmān (Translation of Quran)] (Part 15, Sūraĥ Banī Isrāīl, verse 36)
Allah عَزَّوَجَلَّ says in the 12th verse of Sūraĥ Al-Ḥujurāt (part 26):
یٰۤاَ یُّہَا ا لَّذِ یۡنَ اٰ مَنُو ا اجۡتَنِبُوۡ ا کَثِیۡرً ا مِّنَ ا لظَّنِّ ۫ اِنَّ بَعۡضَ ا لظَّنِّ اِثۡمٌ
O those who believe! Avoid most suspicions; verily some suspicion becomes a sin.
[Kanz-ul-Īmān (Translation of Quran)] (Part 26, Sūraĥ Al-Ḥujurāt, verse 12)
The Prophet of mankind, the Peace of our heart and mind, the most Generous and Kind صَلَّى اللهُ تَعَالٰى عَلَيْهِ وَاٰلِهٖ وَسَلَّم said, ‘(O People) avoid suspicion, because suspicion is the worst lie.’
(Ṣaḥīḥ Bukhārī, vol. 3, pp. 446, Ḥadīš 5143)
The honourable scholars رَحِمَـهُـمُ الـلّٰـهُ الـسَّلَام said, ‘A wicked thought emanates from a wicked heart’.
(Fayḍ-ul-Qadīr Sharḥ Al-Jāmi’-uṣ-Ṣaghīr, vol. 3, pp. 157, Taḥt-al-Ḥadīš 2901)