عن عبد الله بن عمر أنه قال  كنا جلوسا مع رسول الله صلى الله عليه وسلم إذ جاءه رجل من الأنصار فسلم عليه ثم أدبر الأنصاري فقال رسول الله صلى الله عليه وسلم يا أخا الأنصار كيف أخي سعد بن عبادة فقال صالح فقال رسول الله صلى الله عليه وسلم من يعوده منكم فقام وقمنا معه ونحن بضعة عشر ما علينا نعال ولا خفاف ولا قلانس ولا قمص نمشي في تلك السباخ حتى جئناه فاستأخر قومه من حوله حتى دنا رسول الله صلى الله عليه وسلم وأصحابه الذين معه

Sayyiduna ‘Abdullah ibn ‘Umar (radiyallahu’anhuma) reports: ‘We were once seated with Rasulullah (sallallahu’alayhi wasallam) when an Ansari came, said salam on Nabi (sallallahu’alayhi wasallam) and proceeded. Rasulullah (sallallahu’alayhi wasallam) enquired: ‘O brother of the Ansar! How is your brother Sa’d ibn ‘Ubadah?’

He replied: ‘He is fine.’

Nabi (sallallahu’alayhi wasallam) asked: ‘Who among you will visit him?’ He then stood up. We all stood with him. We were more than ten, we didn’t have sandals, leather socks, hats [قلانس] or even shirts. We walked in those salty marshes [in this condition] until we reached him. His family made way and Rasulullah (sallallahu’alayhi wasallam) and his companions came near to him.’

(Sahih Muslim, Hadith:925]
قلانس is the plural of قلنسوة, which is translated as hat or topi.
Shaikh Muhammad Abasoomar comments:

This narration illustrates that not wearing a hat was abnormal and involuntary [as a result of their poverty], just like walking with no shirts or footwear.

See Hadithanswers