SB 10.50.19

śrī-bhagavān uvāca
na vai śūrā vikatthante
 darśayanty eva pauruṣam
na gṛhṇīmo vaco rājann
 āturasya mumūrṣataḥ
Synonyms: 
śrī-bhagavān uvāca — the Supreme Lord said; na — do not; vai — indeed; śūrāḥ — heroes; vikatthante — boast vainly; darśayanti — they show; eva — simply; pauruṣam — their prowess; na gṛhṇīmaḥ — We do not accept; vacaḥ — the words; rājan — O King; āturasya — of one who is mentally agitated; mumūrṣataḥ — who is about to die.
Translation: 
The Supreme Lord said: Real heroes do not simply boast but rather show their prowess in action. We cannot take seriously the words of one who is full of anxiety and who wants to die.