SB 10.50.7-8

haniṣyāmi balaṁ hy etad
 bhuvi bhāraṁ samāhitam
māgadhena samānītaṁ
 vaśyānāṁ sarva-bhūbhujām
akṣauhiṇībhiḥ saṅkhyātaṁ
 bhaṭāśva-ratha-kuñjaraiḥ
māgadhas tu na hantavyo
 bhūyaḥ kartā balodyamam
Synonyms: 
haniṣyāmi — I will kill; balam — army; hi — certainly; etat — this; bhuvi — upon the earth; bhāram — a burden; samāhitam — collected; māgadhena — by the King of Magadha, Jarāsandha; samānītam — brought together; vaśyānām — subservient; sarva — all; bhū-bhujām — of the kings; akṣauhiṇībhiḥ — in akṣauhiṇīs; saṅkhyātam — counted; bhaṭa — (consisting) of foot soldiers; aśva — horses; ratha — chariots; kuñjaraiḥ — and elephants; māgadhaḥ — Jarāsandha; tu — however; na hantavyaḥ — should not be killed; bhūyaḥ — again; kartā — he will make; bala — (to collect) an army; udyamam — the endeavor.
Translation: 
[The Supreme Lord thought:] Since it is such a burden on the earth, I will destroy Jarāsandha’s army, consisting of akṣauhiṇīs of foot soldiers, horses, chariots and elephants, which the King of Magadha has assembled from all subservient kings and brought together here. But Jarāsandha himself should not be killed, since in the future he will certainly assemble another army.
Purport: 

After due consideration, Lord Kṛṣṇa decided that since He had descended to the earth to destroy the demons, and since Jarāsandha was so enthusiastic to bring all the demons to the Lord’s front door, it was definitely more efficient to keep Jarāsandha alive and busy.