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 ﷺ فَرَسًا لأَبِي طَلْحَةَ، يُقَالُ لَهُ: الْمَنْدُوبُ، فَرَكِبَهُ، فَلَمَّا رَجَعَ قَالَ: مَا رَأَيْنَا مِنْ شَيْءٍ، وَإِنْ وَجَدْنَاهُ لَبَحْرًا.