Al-Humaydi reported: Sufyan Ath-Thawri, may Allah have mercy on him, said, “It is said that observing long periods of silence is the key to worship.”
Source: al-Ṣamt li-Ibn Abī Dunyā 433
عَنِ الْحُمَيْدِيِّ عَنْ سُفْيَانَ قَالَ رحمه الله كَانَ يُقَالُ طُولُ الصَّمْتِ مِفْتَاحُ الْعِبَادَةِ
433 الصمت لابن أبي الدنيا