When someone says, "It is a sea" about a man, thing, or horse

Anas ibn Malik, "There was some alarm in Madina and the Prophet, (ﷺ), borrowed a horse belonging to Abu Talha called al-Mandub. He rode it, and when he returned he said, 'We did not see anything and he found the horse to be a sea."

حَدَّثَنَا آدَمُ، قَالَ‏:‏ حَدَّثَنَا شُعْبَةُ، عَنْ قَتَادَةَ قَالَ‏:‏ سَمِعْتُ أَنَسَ بْنَ مَالِكٍ يَقُولُ‏:‏ كَانَ فَزَعٌ بِالْمَدِينَةِ، فَاسْتَعَارَ النَّبِيُّ صلى الله عليه وسلم فَرَسًا لأَبِي طَلْحَةَ، يُقَالُ لَهُ‏:‏ الْمَنْدُوبُ، فَرَكِبَهُ، فَلَمَّا رَجَعَ قَالَ‏:‏ مَا رَأَيْنَا مِنْ شَيْءٍ، وَإِنْ وَجَدْنَاهُ لَبَحْرًا‏.‏