Once, Bhaktāgrima Prabhu, Advaya-jñāna Prabhu, two other brahmacārīs and I went to preach in Shillong. One day, Bhaktāgrima Prabhu went to the shop of a māravāḍī (merchant) for bhikṣā and he gave him five rupees. But Bhaktāgrima Prabhu said that he would not take less than ten rupees and went for bhikṣā elsewhere. When returned, he saw that the shop of that māravāḍī had caught fire and that most of his goods were burned. Bhaktāgrima Prabhu went back to where we were staying and while telling us about the incident said that the māravāḍī had been struck by his curse. Later, when we returned to Śrī Devānanda Gauḍīya Maṭha and told Śrīla Mahārāja (Śrīla Bhaktivedānta Nārāyaṇa Mahārāja) about the preaching, we also told him about that incident. Śrīla Mahārāja listened to everything and did not say a word, but that evening, while giving hari-kathā, he asked everyone, “How can we understand if our heart is becoming purified or not?” When none of us responded, he himself gave the answer, “When we are not disturbed even when all circumstances are there to agitate us, when we are insulted by someone but do not request God to punish that person – meaning we do not curse him. Rather we think that such insult is due to our past actions. Only then can we understand that our heart is being purified.” Through this hari-kathā, Śrīla Mahārāja imparted the teaching that those who desired to become sādhus should not even think of cursing somebody.
Source: Rays of The Harmonist Tirobhāva Edition